1. Field of the Invention
This invention relates to measuring temperature by the use of thermocouples. More particularly, this invention relates to amplifier circuits operable with thermocouples and providing cold junction compensation.
2. Description of the Prior Art
Thermocouples have been employed for many years to measure temperatures in a wide variety of applications. As is well known, thermocouples produce an output voltage proportional to the temperature difference between the measurement junction (which is exposed to the temperature being measured) and a reference junction (often referred to as the "cold junction"). Since changes in the reference junction temperature will alter the thermocouple output, practical installations include means to stabilize the reference temperature, as by placing the reference junction in an ice bath (0.degree. C.), or means to develop a compensation signal which automatically varies with reference junction temperature in such a way as to maintain the output signal constant for a constant measurement temperature. The present invention relates to means for developing such compensation.
Various techniques have been used for effecting cold junction compensation. For example, a voltage may be developed across a resistor in series with the thermocouple output by supplying the resistor with a current having a first component produced by a constant voltage source and a second component produced by a temperature transducer responsive to the reference junction temperature. A temperature transducer which has been successfully used for this purpose is that identified as the Model AD590, manufactured by Analog Devices, Inc. (assignee of this application), and operable to produce an output current proportional to absolute temperature.
For various reasons, the thermocouple amplifier and compensation arrangements used heretofore have not been fully satisfactory. Accordingly, it is a principal object of this invention to provide an improved thermocouple-amplifier-compensator which eliminates or substantially minimizes the disadvantages of the prior art techniques.